The present invention relates to cordierite ceramic and more particularly to an improved method of preparing a precursor or cordierite-forming, particulate raw material mixture, which mixture is useful in making cordierite articles of widely-differing profiles and shapes.
Cordierite ceramic has, among other structural, physical and chemical properties, excellent high temperature capability, chemical durability and resistance to thermal shock, vibration and corrosion. These properties permit the use of cordierite ceramic in a large number of environments and applications, as for example, in filters for fluids, both liquids and gases, as heat exchangers, nozzles, protective linings and as catalyst support members in apparatus for purifying hydrocarbons, carbon monoxide and nitrogen oxide in various exhaust gases.
In general, cordierite ceramic articles are formed by preparing a particulate raw material mixture containing magnesia, alumina, and silica (or their pre-cursors); forming the raw material mixture in a desired configuration; and thereafter sintering or firing the desired configuration to produce the cordierite article. U.S. Pat. Nos. 3,919,384 and 4,551,295, assigned to the same assignee as the subject application, relate, among other things, to the manufacture of cordierite ceramic extruded articles of various geometries.
It is well understood by those skilled in the art that variations in the batch raw materials (i.e. particle size, particle shape, and chemistry of the various raw materials) will introduce product variability in the fired cordierite article. That is, the physical properties of the fired cordierite article will in large part be dependent on the structural, physical, and chemical properties of the batch raw materials.
Consequently, depending on the characteristics of the raw materials which, in turn, are many times dependent on the sources of the raw materials, there will be finished product variations, even though all other process parameters are kept the same.
Two physical properties of cordierite ceramic of particular interest are water absorption and catalyst coatability. With a view of improving both of these properties, the inventor undertook an investigation of the precursor batch raw materials of cordierite ceramic.
It was during a study of the role of the various precursor or cordierite-forming raw materials that the inventor implemented a set of experiments to investigate the role of the magnesia precursor therein. Toward this end, the inventor implemented a set of experiments to examine in detail the specific magnesia precursor talc (3MgO..sub.4 SiO.sub.2.H.sub.2 O). These talc experiments examined the effect, if any, that different talc morphologies have on the physical properties of fired cordierite articles and their water absorption and catalysis coatability properties.
After intensive study, the inventor found that there is a relationship between talc particle morphology, i.e., talc particle surface area, and water absorption values of fired cordierite articles as well as its catalysis coatability.
As will be explained more fully hereinafter in the Detailed Description of the Invention, the inventor developed a Talc Morphology X-ray Diffraction Index, hereinafter alternatively referred to as "TMXDI" or "Index", to quantify talc particle morphology.
It may be explained here that talc particles are sometimes characterized in the prior art as tabular or "platy", or as stone-like or "blocky", depending upon the physical appearance or structure of the talc particles. Such "platy" or "blocky" character, in large part, depends on the source of the ore and the manner in which the ore is processed. As for example, when talc is broken, it can break into sheet-like structures or plate shaped particles along the basal planes (001) which are parallel to the sheet-like or plate shaped particles and perpendicular to the c-crystallgraphic axis. Generally, as the "platy" character of talc particles increases, the particle surface area decreases.
The TMXDI developed by the inventor is a measure of the "platy" character of the talc particles. The TMXDI increases as the talc approaches a more "platy" morphology and can be used to verify variability in the talc surface area. The inventor has determined that there is a direct relationship between water absorption in the fired ceramic cordierite article and the TMXDI, and that there is an inverse relationship of water absorption in the fired ceramic cordierite article to surface area of the precursor talc particles.
Consequently, by characterizing the talc particles in accordance with surface area or with the TMXDI of the present invention, the make-up of the batch raw materials can be more effectively controlled. Use of talc particle surface area or the TMXDI also provides a means to blend talc particle from different sources and suppliers to obtain uniform properties from one production lot to the next. Such control and blending can be a very valuable technique for controlling variability of specific physical properties of the fired ceramic cordierite article, viz., water absorption and catalysis coatability. These factors together with others which will become apparent, are indicative of the invention's advantages.